Investigation into the effect of acid/alkali strength on the heat change when acids and alkalis are mixed
The main aim of this experiment is to investigate the heat change when acids and alkalis are mixed. The temperature of the acid and alkali that are going to be mixed will be taken prior to mixing, and after they have been mixed in a polystyrene cup. The maximum temperature rise will be noted as this will be the biggest heat change that has occurred. The highest temperature after mixing and the temperature prior to mixing will be subtracted to give the heat change.
To ensure that this is a valid test the volume of the acid and alkali will be kept constant at 40cm3. The volume will be kept constant because if there were a differing volume of acid to alkali this would have an influence on the temperature rise as there is not the same amount of solution. The only variable in this experiment will be the strength of the acid and alkali. This will allow us to examine the manner in which the heat evolved differs for differing strengths. Comparisons can then be undertaken to see how concentration affects the heat change in set volumes of acid and alkalis.
When the acid and alkali of the same volume are mixed, this will cause the process of neutralisation to occur. Neutralisation is the reaction between an acid and a base. It is the formation of a bond between H+(aq) from the acid and OH-(aq) from the base. These are known as the reacting ions. This is because in the solution the ions are dissociated and thus independent.
H+(aq) + OH- (aq) H2O (l)
As this is a bond forming process it is known that it will liberate energy, thus all reactions between acids and bases are exothermic.
Diagram of apparatus
40cm3 of hydrochloric acid
40cm3 of ethanoic acid
40cm3 of sodium hydroxide
40cm3 of ammonium hydroxide
The 20cm3 acids (hydrochloric acid and ethanoic acid) will be measured accurately in 50cm3 measuring cylinders. The same will be done for the alkalis. The cylinders should either be marked or have some distinctive feature so that they are not confused for another liquid, thereby causing a mistake to occur, such as the wrong alkali or acid being mixed. Once the liquids have been measured they will be mixed in the following order:
1) Strong acid + Strong alkali
Hydrochloric acid + Sodium hydroxide
2) Strong acid + Weak alkali
Hydrochloric acid + Ammonium hydroxide
3) Weak acid + Strong alkali
Ethanoic acid + Sodium hydroxide
4) Weak acid + weak alkali